To materials engineers, composites offer the ability to alter the properties of materials by combining the functionalities of several components for a specific purpose. It is widely believed that, for example, ceramic/polymer nano-engineered composites can be designed to exhibit the overriding strength, dimensional, and thermal stability of ceramics with the fracture properties, processability, and dielectric properties of polymers. “Matrix Nanocomposites” are a new class of materials, which exhibit ultra-fine dispersed phase dimensions, typically in the 1-100 nm range as well as a physically distinct host or continuous phase over much longer average length scales. The nanolength scale is the range where phenomena associated with atomic and molecular interactions strongly influence the macroscopic properties of materials such as electrical and thermal conductivity, strength, and optical clarity, for example, the longer length scale phase are typically used—in addition to the afore mentioned macroscopic properties—to determine processing and fabrication of the composite.
Preliminary experimental work on nano-composite materials have suggested that many types and classes of nanocomposites have new and improved properties when compared with their macro-scale counterparts (see for example: Ajayan, P. M. Chem. Rev. 1999, 99, 1787). A predominant feature of these materials is their ultra fine phase dimension and hence surface area; therefore a large fraction of the atoms reside at an interface. The properties can therefore be expected to be strongly influenced by the nature of the interface. For example, a strong interface should allow unusual mechanical properties. Since the interface structure plays a critical role in determining the properties of these materials they are frequently referred to as “interface composites”.
To make a successful nanocomposite it is very important to be able to disperse the secondary phase (be it a nanosized metal, ceramic, or polymer) throughout the host material and create those interfaces.
Nanocomposites are now becoming viable commercial products. BIB.: Y. Feng, Y. Ou, Y. Zhongzhen, J. Appl. Polym. Sci., 69(2), 355, 1998.). Most research and development is focused on automotive parts and packaging, and space durable composites (“Fluoropolymer Nanotube Composites for Coatings and Nanoscopic Probes” Shah, H.; Czerw, R.; Carroll, D.; Goldner, L.; Hwang, J.; Ballato, J.; Smith, Jr., D. W. Polym. Mat. Sci. & Eng. (Am. Chem. Soc., Div. PMSE) 2000, 82, 300.), display applications (P. M. Ajayan, O. Stephan, C. Colliex, and D. Trauth, Science, 265, 1212, 1994.), and atomic force microscopy (AFM) probes (Dai, H.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley R. E. Nature, 1996, 384, 147 and Hafner, H. G.; Cheung, C.; Lieber, C. M. Nature, 1999, 398, 761).
The benefits of such nanocomposites that have already been identified include: efficient reinforcement with minimal loss of ductility, improved impact strength, heat stability and flame resistance, improved gas barrier properties, improved abrasion resistance, reduced shrinkage and residual stress, altered electronic and optical properties (See for example: “Handbook of Nanophase Materials” A. N. Goldstein, Ed., Marcel Dekker, Inc., New York, 1997 and S. J Komarnemi, Mater. Chem., 2, 1219, 1992). The shapes of the particles used in nanocomposites can vary from spherical, fibrilar, to platelets, and each will result in different properties modifications to the host. For example: for maximum reinforcement, platelets or fibrilar particles would be used, since reinforcement efficiency tends to scale with the aspect ratio (L/d). Further, performance benefits of nanoparticulate fillers are available without increasing the density or reducing light transmission properties of the base polymer. Although many research projects have been reported concerning all types of nanoparticles, the most extensive research has been performed with layered silicates, which provides platelet reinforcement.
Matrix nanocomposites, based on polymers, have been a central area of polymer research in recent years and significant progress has been made in the formation of various types of polymer-nanocomposites. This includes an understanding of the basic principles that determine their optical, electronic and magnetic properties. An early polymer nanocomposite that was developed was the polyamide 6 (from caprolactam), which has dispersed ion-exchanged montmorillonite, a smectic clay, as the reinforcement. Such nanocomposites typically contain 2-10% loadings on a weight basis, yet property improvements can equal and sometimes exceed traditional polymer composites even containing 20-35% mineral or glass. Machine wear is reduced and processability is better. Because polymers are, typically, about one-half as density as mineral and glass fillers these composites offer attractive opportunities for weight sensitive applications, such as auto parts.
Fluoropolymers are known to represent viable alternatives to current optical materials particularly for the critical next step in optical communications—access level all-optical networks (Modern Fluoropolymers, Scheirs, J., Ed.; Wiley: New York, 1997).
A pending U.S. patent application Ser. No. 09/604,748 entitled “Fluoropolymers and Methods of Applying Fluoropolymers in Molding Processes” and assigned to the assignee of the present application is directed to uses of PFCB compounds in molding processes and optical applications.
Other publications have recited various synthesis methods and uses for PFCB and fluoropolymeric compounds. See, i.e. Smith et al, “Perfluorocyclobutyl Liquid Crystalline Fluoropolymers. Synthesis and Thermal Cyclopolymerization of Bis(trifluorovinyloxy)-alpha-methylstilbene”, Macromolecules, Volume 33, Number 4, Pages 1126-1128; See also Smith et. al. “Perfluorocyclobutane (PFCB) polyaryl ethers: versatile coatings materials”, Journal of Fluorine Chemistry 4310 (2000) 1-9. There is great potential for this optical fluoropolymer to further enhance its properties by using it in a nanocomposite where the nanomaterial provides unique interactions with light.
In regards to electrically conductive polymer composites, work has been done using carbon black as a second phase to permit conductivity in an otherwise dielectric host. See Foulger, Stephen: “Reduced Percolation Thresholds of Immiscible Conductive Blends”, Journal of Polymer Science: Part B: Polymer Physics, Vol. 37, 1899-1910 (1999).
Although carbon black was used in the previous case, many forms of carbon exist. For example carbon may be in the form of submicron graphitic fibrils, sometimes called “vapor grown” carbon fibers. Carbon fibrils are vermicular carbon deposits having diameters less than about 1.0 micrometer. They exist in a variety of forms and have been prepared by catalytic decomposition of carbon-containing gases on metal surfaces.
U.S. Pat. No. 4,663,230 discloses cylindrical ordered graphite cores, uncontaminated with pyrolytic carbon. Blending such fibers with polymers has been known to improve the mechanical properties of the resulting blends.
More recently, it has been found that carbon tubes (often termed “nanotubes”) provide a structure with potential for many such applications. In particular, the structure of carbon nanotubes makes their aspect ratio (length/diameter, L/D) comparable to that of long fibers. Typically the aspect ratio of carbon nanotubes is >10,000. Thus, the aspect ratio of carbon nanotubes is generally much greater than that of conventional short fibers, such as those often made of glass or other forms of carbon. In addition, nanotubes sometimes may be lighter than conventional carbon fibers, which may be helpful in some applications.
Currently, carbon nanofibers and carbon nanotubes figure prominently among the organic-host nanocomposite fillers of interest. Vapor grown carbon nanofibers (VGCFs) in thermoplastic matrices have attracted much interest as they have potential application as conducting polymers for electrostatic dissipative coatings. In addition the VGCFs enhance both stiffness and thermal stability of the matrix. Thermoplastic matrices noted in recent studies include polypropylene, acrylate-butadiene-styrene, polyethylene, polycarbonate and polyether-terephthalate. Naturally, the interactions between the fiber and the matrix at the interfacal level are of critical importance to the properties of the developed composite. The catalytically grown carbon fibers used in these previous studies have outstanding physical properties such as high tensile modulus as well as low electrical resistivity and high thermal conductivity (1,950 W/mK). Further, these nanofibers can be surface-treated to promote different types of bonding. However, electrical and thermal conductivities as well as yield strengths and moduli are orders of magnitude larger for carbon nanotubes and one might want to extend these nanocomposites to include nanotube dispersions instead of VGCFs. This is because network formation at the percolation threshold of nanotubes may be achieved at relatively low mass concentrations because of their unusually high aspect-ratios. Hence, enhanced electrical and thermal conductivity may be possible in polymer/nanotube composites without sacrificing, for example, host optical clarity or flexibility.
Unfortunately, control over dispersive characteristics is significantly more difficult for carbon nanotubes. This comes about because, unlike VGCF, the surfaces of the nanotubes are exceedingly difficult to modify as they exhibit, primarily, unreactive carbon-carbon bonds.
Efforts have been made to incorporate carbon nanotubes into hydrocarbon-based polymeric materials, but difficulty has been encountered in providing compositions that perform well. In general, the number of carbon nanotubes that must be placed into a polymeric composition to achieve superior properties is so high that the actual physical and structural properties of the polymer may be deteriorated by the presence of the carbon nanotubes. This difficulty may be due to the fact that carbon nanotubes tend to clump and aggregate together (instead of uniformly dispersing) when placed in many hydrocarbon-based polymeric compositions.
This relatively poor control over the dispersive characteristics has made it difficult to employ carbon nanotubes in useful applications. The surfaces of such nanotubes may be exceedingly difficult to modify as well, since nanotubes exhibit primarily unreactive carbon-carbon bonds.
What is needed in the industry is a composition and method of preparing a composition that is capable of employing the useful properties of nanomaterial structures in a polymer matrix. A composition that successfully combines nanomaterial structures uniformly dispersed in a polymer matrix would be highly desirable.